In general, a tonic contraction occurs when the number of pulses per second exceeds:

• A. 10
• B. 20
• C. 30
• D. 40

Answer: (C)

A tonic contraction refers to a continuous, steady contraction of a muscle, often resulting from sustained electrical stimulation. This type of contraction is characterized by its ability to maintain tension over an extended period, as opposed to phasic contractions, which are shorter and more intermittent.

The threshold for a tonic contraction typically occurs when the frequency of the pulses exceeds a certain level. In this context, when the number of pulses per second surpasses 30, it leads to a sustained contraction of the muscle fibers. This frequency ensures that the muscle fibers do not have time to relax fully between stimuli, resulting in a continuous contraction that can be maintained for a longer duration.

Thus, when the question indicates that a tonic contraction occurs when the number of pulses per second exceeds 30, it aligns with the principles of muscle physiology concerning the relationship between stimulation frequency and muscle contraction types. Frequencies lower than this would likely result in more phasic contractions, as the muscle would have time to relax between individual stimuli.